Dictionary of Evidence-based Medicine.pdf

o
OBM (see Opinion-based medicine)
Odds ratio
The odds of an outcome (e.g. adverse reaction following exposure to a
drug) is the ratio of the probability of the outcome occurring to the probability
of it not occurring. If the odds (O) of an event occurring following
exposure to drug A is (Oa) and the odds of the same event occurring
following exposure to drug B is (Ob), the ratio of the two odds (Oa/Ob) is
known as the odds ratio (OR).
In the above example, an odds ratio of, say, 5 means that the odds of
having the event in question following exposure to drug A is five times
that following exposure to drug B.
Table 5 Contingency table showing a hypothetical set of outcomes following treatment
of two groups of 100 patients each with two treatments A and B
Treatment A
Treatment B
Event present 15 5
Event absent 85 95
Total events 100 100
The odds of the event happening with treatment A = 15/85 and with
treatment B = 5/95. The odds ratio of the event happening with treatment
A relative to treatment B = (15/85 divided by 5/95) = 2.68. The risk of the
event happening with treatment A = 15/100 and with treatment B = 5/100.
The risk ratio of event occurring with treatment A relative to B = (15/100
divided by 5/100) = 3. The risk difference for the event happening with
the two treatments being compared = (15/100 - 5/100) = 0.1. The number
needed to treat = I/risk difference = 10.